Three-dimensional sound source positioning is gaining more and more interest. This is especially true for the mobile domain. Music playback and sound effects in mobile games can add a significant experience for a consumer when positioned in the three-dimensional space. Traditionally, the three-dimensional positioning employs so-called head-related transfer functions (HRTFs), as described in F. L. Wightman and D. J. Kistler, “Headphone simulation of free-field listening. I. Stimulus synthesis” J. Acoust. Soc. Am., 85:858-867, 1989.
These functions describe a transfer from a certain sound source position to eardrums by means of an impulse response or head-related transfer function.
Within the MPEG standardization body a three-dimensional binaural decoding and rendering method is being standardized. This method comprises generation of a binaural stereo output audio from either a conventional stereo input signal, or from a mono input signal. This so-called binaural decoding method is known from Breebaart, J., Herre, J., Villemoes, L., Jin, C., Kjörling, K., Plogsties, J., Koppens, J. (2006), “Multi-channel goes mobile: MPEG Surround binaural rendering”, Proc. 29th AES conference, Seoul, Korea. In general, the head-related transfer functions as well as their parametric representations vary as a function of an elevation, an azimuth, and a distance. To reduce an amount of measurement data, however, the head-related transfer function parameters are mostly measured at a fixed distance of about 1 to 2 meters. Within the three-dimensional binaural decoder that is being developed, an interface is defined for providing the head-related transfer function parameters to said decoder. In this way, the consumer can select different head-related transfer functions or provide his/her own ones. However, the current interface has a disadvantage that it is defined for a limited set of elevation and/or azimuth parameters only. This means that an effect of positioning sound sources at different distances is not included and the consumer cannot modify the perceived distance of the virtual sound sources. Furthermore, even if the MPEG Surround standard would provide an interface for head-related transfer function parameters for different elevation and distance values, the required measurement data are in many cases not available since HRTFs are in most cases measured at a fixed distance only and their dependence on distance is not known a priori.